The Organic Light Emitting Diodes (OLED) display device possesses many outstanding properties of self-illumination, low driving voltage, high luminescence efficiency, short response time, high clarity and contrast, near 180° view angle, wide range of working temperature, applicability of flexible display and large scale full color display. The OLED is considered as the most potential display device.
The OLED can be categorized into two major types according to the driving methods, which are the Passive Matrix OLED (PMOLED) and the Active Matrix OLED (AMOLED), i.e. two types of the direct addressing and the Thin Film Transistor (TFT) matrix addressing. The AMOLED comprises pixels arranged in array and belongs to active display type, which has high lighting efficiency and is generally utilized for the large scale display devices of high resolution.
The AMOLED is a current driving element. When the electrical current flows through the organic light emitting diode, the organic light emitting diode emits light, and the brightness is determined according to the current flowing through the organic light emitting diode itself. Most of the present Integrated Circuits (IC) only transmit voltage signals. Therefore, the AMOLED pixel driving circuit needs to accomplish the task of converting the voltage signals into the current signals. The traditional AMOLED pixel driving circuit generally is 2T1C, which is a structure comprising two thin film transistors and one capacitor to convert the voltage into the current. The two thin film transistors respectively are the drive thin film transistor and the switch thin film transistor.
For the 2T1C AMOLED pixel driving circuit, the drive thin film transistor and the switch thin film transistor are usually the same type, and both are oxide thin film transistors or both are Poly Silicon thin film transistors. Although the mobility of the Low Temperature Poly Silicon (LTPS) thin film transistor based on the Excimer Laser Annealing (ELA) technology is high but the equality is poor, and the leakage current is higher. On the contrary, the equality of the oxide semiconductor thin film transistor is better, and the leakage current is low but the mobility is lower and the reliability is bad. Therefore, no matter the oxide semiconductor thin film transistor or the Poly Silicon thin film transistor is utilized, there are defects existing in the present 2T1C AMOLED pixel driving circuit.